After the gun attack on US national holiday in a suburb of Chicago, the alleged shooter has been accused of seven murders.
Prosecutor Eric Rinehart said Tuesday in Highland Park, Illinois, that "dozens more" charges against 21-year-old Robert Crimo are likely to be added during the course of the trial.
If convicted, Crimo would have to be imprisoned until the end of his life, Rinehart said.
Investigators are convinced that the 21-year-old opened fire on spectators at an Independence Day parade in Highland Park on Monday with a high-penetration rifle.
The death toll recently rose to seven.
More than 35 people were injured.
Mental problems
According to investigators, Crimo had been planning the crime for weeks and disguised himself as a woman on the day of the attack to conceal his identity and make it easier to escape.
He was eventually arrested hours after the fact after a brief chase.
Investigators say they have no clues as to a motive.
The alleged shooter had mental problems in the past and was noticed by threatening behavior.
In 2019, the police were called twice to Crimo's house, a police spokesman said on Tuesday.
The first time because of a suicide attempt and the second time after Crimo threatened to "kill everyone" in the family, according to a relative.
At that time, the police confiscated 16 knives, a dagger and a sword.